I had this problem with my 90 300D. The PO apparently took it to someone who buggered it hideously. I tried vise grips and a pipe wrench and had a couple of local guys in the neighborhood try it as well, with no success.

I took it to an indy shop and a pro ground down two flat sides and torched it, and spun it off with visegrips in about 5 minutes. The owner told me to pay him whatever I thought was ok, I asked if $10 was fair and he said "sure".

He tightened it finger tight and I took it home and changed the oil and replaced the plug. You want to get a new plug specifically for Mercedes: the fit-all plugs will only hold if screwed on too tight.

It is not likely that a drain plug would have rusted on, being as there is always oil on the engine side

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)